Unfortunately, I had to reformat and lost a shitload of my ytmnd-bookmarked pages, with some great ass ones. Personally, my favorites are:
(In no particular order)
Emo Oranges
Medieval NSMB (in fact, most of the medieval ones)
Picard is a Pervert
Conan is an Alligator
Dr. Dre ft. Cosby (recent--so damn well made imo)
Optimus Prime Having a Wonderful Time
Finnish Duck Tales
Safety Not Guranteed
Dean and Costello do Outkast
My list used to be more extensive, but meh, that's what I have for now. Obviously I didn't include some other great fads/ytmnds like Blue Ball Machine (#1 alltime currently), or facial expressions ones, as priceless as they are. I'm sure the vast majority of you have seen them though =P
I voted for "Ratings" because as you can see in my list, most of them have extremely high ratings / are very popular for the most part, and in some cases (like Safety not Guranteed), fad starters.
EDIT: shit, forgot to put the ffxiidelay one (with stewie's voice, talking to brian about his novel), and my search function is down...